The present invention relates to the field of electropyrotechnic initiators intended to ignite the pyrotechnic charges of the gas generators that have to be activated in motor-vehicle airbags. More specifically, the invention relates to an initiator whose initiation head comprises a thick-film metal multifoil circuit which is well protected against electrostatic discharges.
Conventionally, electropyrotechnic initiators intended for motor-vehicle safety consist of an insulating body extended by a fragmentable metal cap and penetrated by two electrodes. The electrodes are connected together via a resistive heating filament surrounded by an explosive initiating composition, for example a composition based on lead triresorcinate. However, such initiators, which are described for example in United States Patents U.S. Pat. No. 4,517,895 or U.S. Pat. No. 4,959,011, have the drawback of being sensitive to the vibrations of the motor vehicle at the soldered joints between the resistive filament and the electrodes. When these soldered joints are repeatedly stressed by the vibrations of the vehicle, they can break and make the igniter inoperable.
To remedy this drawback, initiators have therefore been developed in which the electrodes are in contact with two separate conducting metal areas lying on the surface of the insulating body which is inside the metal cap. These two areas are connected together via a narrow resistive flat strip deposited on the surface of the insulating body. The conducting areas and the resistive strip are covered with an explosive initiating composition.
These initiators fall into two large families. Firstly, initiators whose conducting areas consist of printed circuits, like those described in European Patent EP 0,802,092 for example, and, secondly, initiators in which the conducting areas and the resistive strip consist of a stack of photoetched metal foils, like those described, for example, in U.S. Pat. No. 5,544,585. Initiators corresponding to the latter family are often called xe2x80x9cthick-film multifoil initiatorsxe2x80x9d, the thickness of each metal foil being generally between 2xc3x9710xe2x88x926 m and 7xc3x9710xe2x88x926 m, that is between 2 and 7 micrometres.
Initiators whose conducting areas consist of printed circuits allow electronic components such as varistors or capacitors which provide the initiator with a very high level of electrostatic protection, to be easily soldered using surface mount technology.
On the other hand, with thick-film multifoil initiators which are, however, simple and inexpensive, this soldering is not so easy and these initiators were, hitherto, less well protected against electrostatic discharges.
The object of the present invention is specifically to provide thick-film multifoil initiators which are well protected against electrostatic discharges while requiring no soldering of electronic components during assembly.
The invention therefore relates to an electropyrotechnic initiator, comprising, inside a fragmentable container closed and supported by an overmoulding, an initiation head consisting of an impermeable wall formed by a solid body of height h which has a plane upper face and which grips, over its entire height h, a glassy structure penetrated by two electrodes in the form of pins, the said electrodes each having one end which protrudes from the said plane upper face, this protrusion allowing them to be electrically connected to an electrical thick-film multifoil circuit supported by an insulating support which rests on the said plane upper face and is itself penetrated by the said electrodes, the said circuit having a flat resistive heating element connected to the said electrodes via two separate conducting metal areas lying on the said support, each area being in contact with one of the two electrodes, the said flat element and the said metal areas being covered with a pyrotechnic initiating composition, the said initiator being characterized in that:
the said thick-film multifoil circuit comprises a first film consisting of a first foil made of a resistive metal alloy, having a thickness of between 2xc3x9710xe2x88x926 m and 7xc3x9710xe2x88x926 m, adhesively bonded to the said support and penetrated by the said electrodes, the said first foil having a shape which has, between the electrodes, a central part forming the flat resistive element and the external outline of which first foil, except for the said central part, consists of curved lines whose radii of curvature are greater than 7xc3x9710xe2x88x924 m, the said first foil being covered, except for the said central part, with a second, conducting, metal film serving to form the metal areas which are penetrated by the said electrodes and which have a shape and thickness which are similar to those of the parts of the first foil that they cover.
According to a first preferred variant of the invention, the shortest distance between the edge of the insulating support and the external outline of the thick-film multifoil circuit is at least 35xc3x9710xe2x88x925 m.
According to a second preferred variant of the invention, the said central part has a constant width.
The initiators according to the invention are very simple and robust, they include no additional component fixed by soldering and are easy to manufacture when the thick-film multifoil circuit is produced by photoetching, as described, for example, in Patent U.S. Pat. No. 5,544,585. It has been found that these initiators exhibit remarkable resistance to electrostatic discharges, particularly when the shape of the thick-film multifoil circuit is such that the plane joining the two electrodes passes, level with the support, through an alternation of conducting zones and insulating zones, as will be described in detail later in the description.
The first foil will advantageously consist of a resistive alloy based on nickel and chromium whereas the second film will advantageously consist of a copper foil.
The second foil may advantageously be covered with a third film which will be a coating of tin-based tinning similar in shape and thickness to the parts of the second film that it will cover.
Finally, the overall shape of the electrical thick-film multifoil circuit will advantageously be that of an xe2x80x9cSxe2x80x9d.
The initiators according to the invention can thus easily be mass-produced at modest cost and their preferred application is in pyrotechnic gas generators intended to activate airbags in a motor vehicle.